Picked up an 8KW natural gas gen set for backup power at my place. It has a 24hp engine. I'm planning to set it near my utility meter, which is about 100ft from my gas meter. It's out of the way and far enough away from the buildings for noise. 1.25" SDR tube? 1"? 1" is the biggest I can easily find. A setup with 2 risers, 100ft of pipe, detect wire, etc is about $240 from Home's Depot. The 1.25" I have to go to a supplier and it's going to run me closer to $600 for the same stuff. Generac used the same engine for the 8 and 10k units, so I'd imagine it's not running full out on the 8kw setup. Even still seems like a large engine for small gen head. My portable gen set is 6hp for 5500 watts! The gas meter supplies a 90k BTU combi boiler (water heater/boiler) and in the future another similar sized unit for a house, and probably gas kitchen stove. Don't know if that matters for pipe sizing?
Post up the Make/Model/Serial if you have it, might be able to dig up the specific install manual. A quick look at a Generac Generator Sizing Guide shows for 7-8kW, 1" pipe is good up to a 150ft run on NG running 5-7"WC. Add 8ft to OAL for each 90 degree fitting installed.
Run straight back to the meter and you don't need to consider any other loads. We tried the HD stuff, it is garbage. Get the real stuff from a supplier with the large push-in connections and get the proper tools and follow the instructions.
Another thing to check is how much your gas meter can flow. At least here, the standard size meter almost always has to be upsized to flow enough for a genset on top of the other gas appliances.
Gas meters can generally flow 2x their rating without a problem. Genset that small will not be a problem on a standard 250 class meter on NG. We run 20-24 kw off them all the time, along with the rest of a gas powered house.